2.1 Laser scanning planning
Given the fine geometric details on the artefact and the resolution required, we chose to use a triangulation-based scanning device.
Active triangulation scanners generally have multiple lenses sets or camera settings, to cover areas of different sizes with different
resolutions. These arearesolution setups are just a finite number and the selection depends on the characteristics of the object, on
the sampling resolution needed, and the practical constraints of the working setup. A smaller area covered in each scan means a
higher sampling resolution, but also a more fragmented digitization i.e. more acquisitions, which requires more time to
be sampled and more alignment work. Conversely, a wider area covered in each scan produce a lower sampling resolution, but
the surface coverage is quicker, and the alignment possibly faster. Following these considerations and given the short time available
to carry out the digitization, the CNR-ISTI team employed a Konica-Minolta Vivid 910, a triangulation-based laser scanner
Fig. 3a and b. Two different lens sets were used:
wide
covering ca 50x40 cm in each shot for the less detailed and lower area of
the Sarcophagus and
medium
ca 30x20 cm shot coverage for the details and human figures. Even though it was not a very
“recent” instrument, it was still the most versatile device in its category and it was able to work at the required resolution.
During the planning, possible arising problems were also considered:
-
attainment of a rigid alignment
: range scans obtained with a triangulation scanner are generally aligned only using geometric
redundancy of overlapping scans. Since the lower part of the Sarcophagus body is pretty featureless, we feared that a good
alignment of scans might not be possible in case of small coverageoverlap. On the contrary, if each scan covers a wider
area, this would result in a more stable and rigid alignment since the overlapping regions would have been larger. On the other
hand, the human figures have plenty of geometrical features, making the alignment easier and certainly more rigid.
-
mixing of various resolutions
: while the aforementioned choice is sound considering time constraints and registration issues, the
mixing of two large chunks of data digitized at a different resolution requires some care, as the alignment between the two
parts may be tricky and the final merging may still show a different “roughness” of the 3D surface. However, we evaluated
that the ratio between the two resolution sampling densities was below 1:2, therefore a still manageable level.
-
possible global deformation
of the final reconstructed model: if the range scans are very small with respect of the total size of the
artwork, the alignment error accumulates among the range scans. This could create some degree of deformation of the
reconstructed mesh. Ideally, the larger and fewer the scans are, the more rigid is the final alignment. In a normal situation, to
prevent warping and deformations, external rigid references should have been placed and used in the alignment phase, e.g.:
using external markers, or having a single scan covering the whole object, taken for example with a terrestrial laser scanner
Callieri et al., 2011. In this case, however, time constraints prevented such strategy.
2.2
Photogrammetric planning
From a photogrammetric point of view, image acquisition planning encloses camera network design Fraser, 1996 as well
as rough predictions on photographic settings to use, such as exposure time, sensitivity settings ISO, aperture value and hence
DOF. This permits to select the appropriate lighting power, diffuser, stands etc.
A safe distance from the object and the necessary ground sample distance GSD for the image texture spatial resolution ruled the
camera network planning. A digital single-lens reflex DSLR Nikon D3X 24 megapixel camera featuring a 6 micron pixel size,
coupled with a Nikkor 50 mm f1.8 D lens, was chosen for the acquisition of 14-bit images. Focused at 1 m and using the
nominal value of the focal length, the GSD resulted 0.12 mm on the sharp focus plane 1m. The main body of the Sarcophagus,
i.e. the couch, was then planned to be acquired with an aerial-like camera network consisting of several strips overlapping some
80 along- and 60 across-track, keeping the camera optical axis orthogonal to a hypothetical average object surface within
the field of view. Additionally, rolled and convergent images were planned to strengthen the geometry and help getting more
accurate calibration parameters during self-calibration Nocerino et al., 2014. The approach we generally use is similar to the one
shown in Alsadik et al. 2013, where a virtual cage of cameras can be thought around the object and then specific camera
positions excluded if not satisfying certain criteria. In general, only images outside the convex hull of the asset and its ground
encumbrance were considered. For highly curled or recessed parts that stretch in a specific
direction, such as the garments and braids, images with strips parallel to that direction were planned in order to see the same
part inside the curled area in at least two images. As far as lighting is concerned Fig. 3d and e, a rigorous
approach should take into account several factors such as light positions, their size and orientations for each photograph. To
avoid casting shadows visible in the images, the source of light should originate exactly from the centre of perspective of the
objective lens, i.e. an impossible condition to be realized in practice. Therefore, lights must be positioned symmetrically with
respect to the optical axis of the camera. Under this condition the shadows projected by a lamp or strobe can be filled from the other
symmetrical source of light. Diffuse light works similarly, being the source of light virtually everywhere around the object.
Furthermore cross polarization of the light was used for the acquisition. Cross polarization is a very diffuse technique in the
field of optical microscopes where polarized light is used as a contrast-enhancing technique. On the contrary it is not very
popular in photogrammetry and related disciplines as it requires a controlled light environment adding also some extra time for a
proper tuning of light and filters for each photographs. As advantage it gives images virtually free from specular reflection
components leaving thus only the diffuse one. Colours are more saturated pure and lighting more homogeneous all over the
photographic subject.
3. ACQUISITIONS
When planning the time needed for data acquisition, temperature acclimatization of the equipment must be considered as it may
take some hours for the scale bars, scanners and cameras to reach the ambient temperature.
Calibration of triangulation scanners is often a mandatory procedure to be accomplished before the survey starts. Some
scanners just have a self-calibration phase or do not require it at all, due to the mechanical stability of their components.
The need of camera calibration for photogrammetric measurements Remondino and Fraser, 2006 is today more and
more discussed. If a good camera network can be achieved, self- calibration should not differ from calibration on a fixture with
targets. Nevertheless it is still broadly accepted as good practice to perform at least one calibration just before andor after the
survey to check the camera calibration parameters have not changed significantly. In case of restricted acquisition time, it is
better to have real-time feedback to make sure everything is
This contribution has been peer-reviewed. doi:10.5194isprsarchives-XLI-B5-675-2016
677
a b
c
d e
f Figure 3. Data acquisition: the adopted triangulation-based laser scanner, a Konica-Minolta Vivid 910 a, b; Gretag Macbeth colour checker c;
photogrammetric survey with artificial light and camera mounted on a tripod d,e,f.
going smoothly as planned. For laser scanning this may be accomplished by checking and aligning each scan right after they
are acquired. With old generation hardwaresoftware, this required one of the surveying personnel to be dedicated to this
task, whereas today many scanning software provide real-time feedback on data quality and on-the-fly alignment. From a
photogrammetric point of view, a real-time image checking and processing could be achieved working in tethering mode camera
connected to a laptop or by removing the memory cards or using in-camera Wi-FiBluetooth capabilities or Wi-Fi memory cards.
Once the images are on a laptop, an adjustment procedure can be performed on low-resolution images to check the acquired data.
3.1
Laser scanning acquisition
The surface of the artefact was not particularly difficult for the scanner. However, some areas were a bit shinier and some other
slightly darker than the rest, requiring a tuning of the scanner parameters. The scanning took 5 hours, producing 66 scans
acquired with the wide-area lens and 285 scans acquired with the medium-area lens. The digitization strategy for this kind of
artworks is pretty straightforward following standard, common sense, considerations: the scanner is moved around the artefact,
trying to keep a somehow constant distance from the subject it depends on the device and lenses used, in this case, around 70-
80cm, a straight view-direction with respect to the surface and enough overlap among the range maps. The reason is twofold: i
a sufficient overlap is necessary in the alignment step, as alignment rely on the geometry of the common area between
scans; ii enough overlap means that, probably, most of the
subject’s surface is digitized from more than one direction and this may reduce the sampling noise when merging the data
notwithstanding an increase of the overall sampling density. Since obtaining a direct feedback on the degree of overlap from
the digitization software is not always possible, the correct amount of overlap must be evaluated by the operator, at the
moment of deciding the next shoot. As a rule of thumb, overlap should never be less than 25, and a good interval is between
30 and 50 for difficult areas. When switching from one lens set to the other, great care was
posed in order to have a pretty large overlap between the two areas, to ensure a good alignment between the two datasets.
Figures 4a and b show the area covered by the wide and medium lenses. A photographic acquisition was also required for texture
mapping purposes and it was carried out in a couple of hours of the next morning.
Since the light sources we had could not completely overcome the existing lighting coming from two opposing windows, we
decide to exploit it, and just try to make it more uniform.
a
c b
Figure 4. Laser scans acquired with the wide a and medium lenses b. Photogrammetric camera network c: images acquired for the main block are displayed in blue, images acquired
for the most difficult parts spouses’ trunks, hands, feet are in red.
This contribution has been peer-reviewed. doi:10.5194isprsarchives-XLI-B5-675-2016
678
As the light coming from the windows was already “diffuse” and not “direct” a direct, strong lighting is not suitable for texture
mapping, we used the camera-mounted flash directing it towards the ceiling. This bounced light, added to the ambient lighting,
produced a quite smooth illumination. All photos were radiometrically calibrated using a Gretag Macbeth colour
checker Fig. 3c. 3.2
Photogrammetric acquisition
Given the low budget and strict timing at disposal for the project, only a lightweight photographic gear was selected. The whole
gear fit in two regular suitcases that two people carried on public transportations. The 3DOM-FBK team was allowed to leave the
photogrammetric gear in the Etruscan museum of Villa Giulia the day before the survey, for allowing equipment acclimatization.
Before starting the survey some tests were carried out on sample surfaces that were considered more problematic due to their
glossy appearance or lack of texture. Polarized filters were mounted on both lamps and camera.
Figures 5 shows the effect of an image acquired with and without cross polarization. The improvement provided to image quality
by increasing the contrast and removing specular reflection is evident. Artificial lights in the room were switched off and the
influence of ambient light was evaluated shooting with and without the photographic lamps switched on. In our case the
ambient light contribution was mostly non influential and when it was the case due to sunlight entering the windows it was shaded
with a panel. The exposure was measured on a grey card, manually set to 1 sec at f16 ISO 400 and kept almost for the
whole acquisition. The computed DOF using a circle of confusion of 3 pixels corresponding to a GSD of about 0.3 mm
was some 22 cm. Throughout the survey the distance of 1m from the object was checked both using a measuring tape and the in
camera confirmation focus function in the viewfinder.
Figure 5. A test image of the hands and a scale bar acquired with left and without right cross polarization.
As mentioned, for the lower part of the artefact, an aerial-like camera network was used. After each strip, one operator set up
the tripods and lights at the new height while the other one downloaded the images and ran a photogrammetric adjustment
on the collected data. The onsite processing and analyses were performed on a Dell Precision M4600 portable workstation.
The overall acquired camera network is shown in Figure 4c, where the images belonging to the main block are coloured in
blue whereas the images depicting the details and figures are presented in red.
In order to scale the photogrammetric results, being a small artefact with no possibility to use ground control points GCP,
reference scale bars were adopted. It is generally advisable to use more than a single scale bar or to move it in the measurement
volume to have a redundant check. For the Sarcophagus survey, only one lightweight aluminium scale bar, 316.5 mm long, was
employed Figure 5. The size was decided to facilitate its transportation and, above all, to move it in an easier and safer
way close to the masterpiece. The scale bar was imaged in different positions all around the Sarcophagus, taking care of
acquiring it at least once in each image group. The whole survey took approximately 13 hours in two days 8+5 hours ending up
with about 520 photographs. A camera calibration was carried out at the end of the survey using printed photogrammetric coded
targets.
4. DATA PROCESSING
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